The invention is in the field of flow meters, more particularly mass flow meters of the Coriolis type.
Coriolis-type meters are based on the physical principle of conservation of angular momentum as it applies to the Coriolis acceleration of a fluid flowing through a conduit. For example, as illustrated in Sipin U.S. Pat. No. 3,485,098, when fluid flows through a bent tube which is mechanically oscillated about an axis passing through the inlet and outlet ends of the tube, Coriolis forces are generated causing the tube to twist about a response axis. Mass flow aate can be deduced from measuring the degree of this twist.
In Coriolis-type meters an electromagnetic oscillator can drive the tube about its oscillation axis at the system's resonant frequency, thereby producing a Coriolis acceleration and resulting force. The force acts perpendicular to the flow path and in alternate opposite directions as between the two legs of the tube, superimposing an oscillating moment about the response axis on the motion about the oscillation axis. The resulting moment, acting about the response axis and in a plane perpendicular to the driving moment, produces twisting at a deflection angle which is approximately proportional to the mass flow rate for a constant angular velocity. Control over variations in angular velocity can be attempted by a detection scheme which senses the deflection angle near the central position of the tube excursions, i.e. where the angular acceleration of the tube is near zero, at the point of constant angular velocity. The tube can be oscillated relative to a leaf spring of a similar mass, to make use of a convenient resonant frequency, or relative to a parallel tube carrying the same or shared fluid flow.
Various tube shapes have been proposed in the past. For example said Sipin U.S. Pat. No. 3,485,098 shows a tube having a U-shaped operative portion while Sipin U.S. Pat. No. 4,559,833 illustrates an S-shaped conduit. Smith U.S. Pat. No. Re. 31,450 illustrates a U-shaped tube supported at its inlet and outlet ends and oscillated relative to a leaf spring about an axis perpendicular the legs at the support points. In addition, said U.S. Pat. No. Re. 31,450 and Cox et al. U.S. Pat. No. 4,127,028 illustrate a tube shape which also is U-shaped but its legs are closer to each other at their inlet and outlet ends than at the curve of the U. Still in addition, said Cox et al. patent illustrates in FIGS. 4 and 5 a generally O-shaped tube having an inlet and an outlet which in FIG. 4 are approximately radial, and in FIG. 5 are approximately tangential (but in different planes). Two U-shaped, or generally U-shaped, tubes carrying the same or shared flow are illustrated in said Cox et al. patent and in Smith et al. U.S. Pat. No. 4,491,025.
Various techniques have been proposed for deducing mass flow rate from measurements of the effect of Coriolis forces on the tube or tubes. For example, said Sipin U.S. Pat. No. 3,485,098 discusses using strain gauges or magnetic vibration velocity sensors to derive electrical signals related to the motion of the vibrated tube, noting in connection with velocity sensors that their differential output is proportional to mass flow rate. Said Smith U.S. Pat. No. Re. 31,450 states that while there may be worthwhile information obtained by measurements as in said Sipin U.S. Pat. No. 3,485,098, velocity sensors require measurement of a minute differential velocity superimposed on the very large pipe oscillation velocity. U.S. Pat. No. Re. 31,450 therefore forsakes the use of velocity sensors in favor of optical sensors (photo-interrupters) which have a flag (an opaque plate) affixed to the oscillated tube and a photocell and a light source affixed to a stationary frame such that the sensor would detect the passage of a tube leg through a plane fixed in space but would not detect any other aspect of the tube movement. The time lag between the respective passage of each leg of the tube through a respective plane fixed in space, is proposed as a measure of mass flow rate. A later Smith et al. U.S. Pat. No. 4,442,338 proposes the use of velocity sensors (despite the comments on such sensors in U.S. Pat. No. Re. 31,450) or strain gauges, or acceleration sensors. It proposes squaring the sinusoidal outputs of the velocity sensors to obtain the exact same square waves as in said earlier U.S. Pat. No. Re. 31,450, and deducing mass flow rate in the same manner.
Additional examples of material concerning mass flow meters can be found in Young, A. M., "Coriolis-Based Mass Flow Measurement," Sensors, Dec. 1985, Vol. 2 No. 12, pp. 6-10; "Mass Flow Meters," Measurements & Control, Sept. 1985, pp. 295-302; Spitzer, D. W., "Mass Flowmeters," Industrial Flow Measurement, IRP Student Text, Section 12, pp. 133-141; Hickl, E. L. et al., "Mass Flow Measurement In The 80's," , pp. 49-52; "Mass Flow Meters," Section 13, pp. 141-157; "Mass Flowmeter Accurate To.+-.0.15%, " Chemical Engineering, Dec. 10, 1984; "Flowmeter Installs Directly In-Line With Process Piping," Chemical Processing, Mid-Nov. 1984, p. 82, DeCarlo, J. P., "Mass-Flow Measurement," Fundamentals Of Flow Measurement, 1984, Unit 11, pp. 203-220; Plache, K. O., "Coriolis/Gyroscopic Flow Meter," Mechanical Engineering, Mar. 1979, pp. 36-41.
It is believed that a substantial need still remains to suppress undesirable characteristics of the known Coriolis-type mass flow rate meters and enhance desirable characteristics, and this invention is directed to meeting that need.
In one exemplary embodiment, a mass flow rate meter in accordance with the invention uses a pair of generally Omega-shaped conduit, and deduces mass flow rate from changes in the phase difference of signals derived from velocity sensors responsive to relative motion between the sides of the two moving conduits.
A meter embodying this example of the invention brings about significant and surprising advantages in accuracy, ease of manufacture and use, and other desirable characteristics as compared with the known prior proposals, such as the use of U-shaped tube and time lag measurement of the passage of the sides of a U-shaped tube through planes fixed in space proposed in said U.S. Pat. Re. No. 31,450. For example, other things being equal, this example of the invention has a lesser dimension in the direction transverse to the general direction of the incoming and outgoing flow. This can be important in practice because a common application of Coriolis-type meters is in a continuous process, and plant pipe systems tend to be more cramped by parallel pipes in the general direction of process flow, but it is usually convenient to replace a section of pipe by a meter having the dimensions and configuration of an Omega-shaped meter. Another significant advantage is in the type of loading at the points where the inlet and outlet ends of each meter conduit are affixed to supports. Each meter conduit typically is welded to one or more fixed supports at its inlet and outlet ends. These welds can be a weak point. In the driving mode a U-shaped conduit stresses the joint weld in bending stress while an Omega-shape meter stresses it in torsion, which is less likely to cause joint failure. In the response mode, a U-shaped meter stresses the weld joints in torsion while an Omega-shaped meter stresses it in bending. However, in the response mode the bending stress on the weld joints of the Omega-shaped meter is lower because while in the U-shaped conduit the twisting of the curve of the U is transmitted directly to the weld joint by the straight or substantially straight legs, in an Omega-shaped meter it is transmitted through continuously and smoothly curved portions which themselves twist and thus substantially reduce the bending load at the weld joints. The response mode movement in the Omega-shaped meter is small as compared to the drive mode movement, making the bending stress less important. Additionally, the response/drive frequency ratio in an Omega-shaped meter embodying an example of the invention can be about 1.5, which is considerably lower than in a U-shaped
meter. Ratios in the range of about. 1.4 to about 1.6 are contemplated. This lower response/drive frequency ratio brings about the advantages: that there is more phase shift for a given mass flow rate and because twisting is easier about the torsional response axis. Still in addition, in an Omega-shaped meter the conduit is continuously and smoothly curved, to reduce flow resistance and disturbance. For example, the bend radius of a conduit in an Omega-shaped meter embodying an example of the invention preferably is about 2 to 3 times the outside diameter of the conduit, thus assuring low resistance to flow, and low pressure drop across the meter. In contrast, it is believed that other things being the same, the sharp inlet and outlet angles in a U-shaped tube cause more flow resistance and more pressure drop, and the long bend of the U also causes more pressure drop. In addition, it has been found that the shape of the conduit in an Omega-shaped meter inherently increases sensitivity as compared with known U-shape designs; this means that for a given level of sensitivity an Omega-shape meter can use larger diameter conduits, and thus can have less flow resistance and less pressure drop. The advantages of the Omega-shaped meter as opposed to prior proposals such as for an S-shaped meter include, among other things, (i) the fact that the inlet and outlet sections are coaxial, which is desirable when the meter is used in a facility where it is spliced in a straight pipe, and (ii) that an Omega-shape meter can much more effectively eliminate any adverse effect of gas bubbles which may form in a liquid flow, and of liquid (such as a condensate) which may form in a gas flow. The advantages over prior proposals such as the use of generally O-shaped tubes include the fact that if the inlet and outlet are approximately radial, as in FIG. 4 of said Cox et al. U.S. Pat. No. 4,127,028, the joint weld and flow resistance and disturbance problems are similar to those discussed in connection with U-shaped tubes, and if the inlet and outlet are approximately tangential, as in FIG. 5 of the same Cox et al. patent, the conduit is not planar and extraneous vibrational modes can be induced. Other significant and surprising advantages will become apparent from the detailed disclosure below of an exemplary Omega-shape meter.
In an exemplary embodiment of the invention, two generally Omega-shaped conduits share the flow. With liquid they typically are used upside down, i.e. the inlet and outlet sections are at the highest level, so that the conduits would not entrap gas. With gas they are right-side-up, i.e. the inlet and outlet are at the lowest level, so than liquid (such as condensate) can run out. When at rest, the two conduits conform to two substantially parallel vertical planes, and are aligned with each other. Each conduit comprises, in sequence along the fluid flow therethrough, (1) a substantially straight inlet section which has an inlet end secured to an inlet support, as by a welded joint, and extending therefrom and to the right (when viewed from direction normal to the parallel planes), (2) a first bent section which bends clockwise and ends up pointing to the left, (3) a first intermediate section which extends to the left, (4) a second bent section which bends counterclockwise and ends up pointing to the right, (5) a substantially straight middle section which extends to the right, (6) a third bent section which bends counterclockwise and ends up pointing to the left, (7) a second intermediate section which extends to the left, (8) a fourth bent section which bends clockwise and ends up pointing to the right, and (9) a substantially straight outlet section which extends to the right and terminates in an outlet end secured, as by a welded joint, to an outlet support. Braces affixed to the conduits (as by welding) inboard of their inlets and outlets, constrain their relative motion.
A driver alternately pushes apart and pulls together the centers of the middle sections of the conduits at a driving frequency which corresponds to the natural vibration frequency of the system, to thereby oscillate each conduit about a respective oscillation axis which is substantially concentric with that conduit's inlet and outlet sections. If there is no fluid flow through the conduits, their middle sections tend to remain substantially parallel despite their driving mode oscillation. If there is fluid flow through them, the Coriolis forces twist each conduit about a respective response axis which is perpendicular to its oscillation axis and its middle section, superimposing this twisting motion on the driving mode oscillation. The conduits twist about their respective response axis out of phase with each other, i.e. while their second bend sections (their left sides) move toward each other their third bent sections (their right sides) move away from each other, and while their second bent sections move away from each other their third bent sections move toward each other. Sensors at the second and third bent sections of the conduits produce sensor signals related to the relative movement of the second and third bent sections of the two conduits toward and away from each other. The parameter of interest is the change in phase difference between the sensor signal for the second bend and the sensor signal for the third bend. Mass flow rate is deduced from this change in phase difference.